Method and apparatus for preheating air



Oct. 29, 1963 Jg c. AGARw/u.l 3,108,790

- METHOD AND APEARATUS FOR PREHEATING AIR FiledFeb, 20, 1961 FuelINVENTOR A Harney JAGD/SH C. AGARWALv United States Patent O 3,108,790METHD AND APPARATUS FR PREHEATING AIR Iagdish C. Agar-wai, Penn HiiisTownship, Ailegheny County, Pa., assigner to United States SteelCorporation, a corporation of New .Iersey Filed Feb. 20, 19431, Ser. No.90,427 7 Claims. (Cl. 263-19) This invention relates to an improvedmethod and apparatus for preheating air introduced to a blast furnace.

Conventionally a blast furnace is equipped with stoves (usually three)which act as regenerators for preheating the air. Normally one stove ison blast, that is, actually preheating air for the furnace, while theothers are on gas, that is, gaseous fuel is burning in these stoves toraise their temperature. Periodically the stoves are changed over fromone to the other. As a stove continues on blast, its temperature and thetemperature to which it is capable of heating air of courseprogressively decrease. Desirably the blast reaches the furnace at aconstant temperature; hence the usual practice is to bypass a decreasingportion of the air around the stove and mix the bypassed and heated airahead of the furnace. Automatic regulators are known for controlling theportion bypassed to maintain a constant blast temperature. Thisarrangement has a disadvantage that the blast never is heated to themaximum possible temperature. The purpose in preheating the blast is tointensify and accelerate the burning of coke in the furnace with aconsequent reduction of the quantity of coke required. Increasing theblast temperature saves more coke.

An object of my invention is to provide an improved method and apparatusfor preheating air in which the emperature is maintained constant but ata higher level than Vwith conventional arrangements.

A further object is to provide an improved preheating method andapparatus which utilize a direct-fired recuperator in parallel with theusual stoves and bypasses for producing a blast of constant but highertemperature than that otherwise attained.

In the drawing:

FIGURE 1 is a schematic showing of a preheating apparatus constructed inaccordance with my invention; and

FIGURE 2 is a similar schematic showing of a modification.

FIGURE l shows a blast furnace stove 10, a blower 12, a cold blast main13 connecting the blower and stove, and a hot blast main 14 connectingthe stove and a blast furnace, not shown. A bypass 16 is connectedbetween the cold and hot blast mains 13 and 14 around the stove. Thebypass contains an adjustable valve 17 which has an operating means 18.An automatic temperature-responsive regulator 19 is connected into thehot blast main 14 and to the valve operating means 18 to adjust valve17. The adjustment is such that when the stove is on blast the bypassalways carries the proper quantity of air around the stove to maintain aconstant hot blast temperature despite a continually dropping stovetemperature. The structure thus far described is conventional; hence nomore detailed showing is deemed necessary. This arrangement isduplicated for each of the several stoves associated with the furnace.

In accordance with my invention, I connect a line 20 into the cold blastmain 13, preferably ahead of the connection of the bypass 16 to thismain. Line 20 leads to a direct-fired recuperator 21. I connect anotherline 22 between the recuperator and the bypass 16. Thus the recuperatoris connected in parallel with both the stove and bypass 16. Linecontains an adjustable valve 23 which has an operating means 24. Anautomatic 3,108,700 Patented Oct. 29, 1963 ICC How-responsive regulator25 is connected into line 20 and to the valve operating means 24 formaintaining a constant rate of flow through the line. I introducegaseous or liquid fuel along with air for its combustion to recuperator21, the fuel line being indicated at 26. Line 26 contains an adjustablevalve 27 which has an operating means 28 for controlling the flow offuel into the recuperator. An automatic temperature-responsive regulator29 is connected into line 22. and to the operating means 2S foradjusting the quantity of fuel to maintain air leaving the recuperatorat a predetermined temperature. With an appropriate valve arrangement, asingle recuperator 21 can serve several stoves. The individualcomponents of my apparatus are commercially available devices, per senot of my invention; hence no more detailed showing is deemed necessary.

FIGURE 2 shows a modification in which I place a second direct-firedrecuperator 30 in the cold blast main 13 ahead of the connection withline 20. Recuperator 30 has a fuel line 31 which is equipped with atemperatureresponsive control similar to that of the other recuperator21. In other respects this modification is similar to the apparatusshown in FIGURE 1; hence the description is not repeated.

According to my preheating method, I route a controiled constant firstportion of the air through the recuperator 21. This portion constitutesabout 20 to 55 percent of the total quantity of air which passes fromblower 12 into the cold blast main 13. Valve 23, its operating mechanism24, and the flow-responsive regulator 2S automatically maintain thisportion at the desired level. I route a second portion of the airthrough stove 10. This second portion constitutes about to 80 percent ofthe total quantity of air. I route the remaining air, if any, throughthe bypass 16. Valve 17, its operating means 1S and thetemperature-responsive regulator 19 control variations in flow of thesecond and third portions to maintain constant temperature. The maximumtemperature of the first portion is limited only by materials availablefor construction of the direct-fired recuperator; in practice I controlthis temperature to the range 1300 to 1600 F. or preferably 1300 to 1500F. When using the modication of FIGURE 2, I heat the cold blast in therecuperator 30 to temperature of 400 to 800 F. or preferably 500 to 700F. Thus I reduce the heat duty of stove 10 and recuperator 21.

As a specific example of a conventional method of heating air, I heatedair at a wind rate of 120,000 c.f.m. in the usual 3-stove arrangement.Each stove was on gas 118 minutes, and the burning rate in itscombustion chamber was 24,000 c.f.m. of blast furnace gas having aheating value of 92 B.t.u. per cu. ft. Each stove was on blast for 64minutes. The maximum constant blast temperature I could attain was 1700F.

I then incorporated the apparatus of the present invention in theforegoinT arrangement and applied my heating method with the same windrate. In contrast I obtained -a constant hot blast temperature of 1980F. I achieved the higher temperature by routing 20 percent (24,000c.f.m.) of the cold blast through recuperator 21, wherein I heated thisair to 1400 F. I maintained a constant rate of iiow (through therecuperator, but I varied the cold blast passing through the stove from86,000 c.f.m. at the beginning of the on-blast period to 93,400 c.f.m.at the end. `Only 10,000 c.f.m. of cold blast bypassed the stove and.recuperator at the beginning and 2000 c.f.m. at the end. The on-blastperiod lasted minutes.

In another example I used only two stoves. Each stove Was on lgas 56minutes, and the burning rate in the combustion chamber and the windrate Were the same as in sacarse the previous example. Each stove alsowas on blast 56 minutes. I controlled flow through reouperator 2t at aconstant rate of 612,600 c.f.m. or 52.2 percent of total ilow, `andheated this ilow to a constant temperature of 1400 F. At the beginningof the on-blast period the ilow through the stove was 47,400 Cim. (39.5percent), While .10,000 c.f.m. bypassed the recuperator and stove.Operating under these conditions, l was able to achieve a constant blasttemperature of 1680 F., contrasted with a maximum constant temperatureof only 1060l F. by operating the two stoves in the conventional manner.

In Ia third example I used the two-stove arrangement with a lower windrate of 70,000 csfim. By operating in the conventional manner, Iachieved a maximum constant blast temperature of only 1670 F., and byusing my method a temperature of `1940 F. I maintained how through therecuperator at 2.0 percent or 14,000 c.f.m., wherein I heated the air to1600 F. At the beginning of the ort-blast period 67,-8 percent of47,1i00 c.f.-m, of the cold blast passed through the stove, while 12percent bypassed the stove and recuperator.

From the foregoing description and examples, it is seen my inventionaffords a simple method and apparatus for raising the temperature of thehot blast to a blast furnace and yet maintaining this temperatureconstant. aware of previous arrangements which embody a recuperator inseries `'with a stove or replace the stoves entirely with recuperators,but such arrangements have not accomplished comparable results.

While I have shown and described certain preferred embodiments of myinvention, it is apparent that other modifications may arise. Therefore,I do not Wish to be limited to the disclosure set forth but only by thescope of the appended claims. l

I claim:

1. A method of prelieating an air blast comprising routing a constantfirst portion of a cold blast through a recuperator maintained at aconstant temperature, routing variable second and third portions ot' thecold blast through a regenerator yand a bypass respectively, therecuperator, regenerator and bypass being arranged in parallel,combiningthe three portions to form a single hot blast, and relativelyincreasing the second portion land decreasing the third portion as theregenerator becomes progressively cooler to :maintain the hot blast at aconstant temperature. f

2. A method of preheating an air blast comprising routing a constant rstportion of about to 55 percent of a cold blast through a recuperatormaintained atk a constant temperature of about 1300 to 1600 F., routingl amvariable second and third portions ofthe cold blast through arelgenerator and a bypass respectively, the recuperator, regenerator andbypass being arranged in parallel, combining the three portions to'forrna single hot blast, and

relatively increasing the second portion and Ydecreasing the thirdportion as the regenerator becomes progressively cooler to maintain thehot blast at a constanttemperature. u

3. A method as delined in claim 2 in which the hot blast temperature isin the range of 1800 to 2000 F.

4. A method as dened in claim 2 in which the cold blast is heated to atemperature of 400l to 800 F. before the different portions are routedtherefrom.

5. An apparatus -for preheating an air blast to a constant temperaturecomprising a cold blast main, a recur erator connected to said main,means to deliver a constant first portion of the air `from said main tosaid recuperator to raise the temperature of said first portion to a lconstant level, a regenerator and a bypass connected to said main andyadapted to receive variable second and third portions of the aircarried thereby and raise the temperature of the combined second andthird portions to a constant level, and a hot blast main connected tosaid recuperator, regenenator and bypass for receiving a combinedconstant temperature hot blast therefrom.

6. An apparatus for pre/heating an air blast to a constant temperaturecomprising a cold blast main, a direct-fired rccuperator connected tosaid main, Imeans to deliver a constant first portion of the air fromsaidmain to said recuperator, means for controlling the temperature ofsaid recuperator to maintain at a constant level the temperature towhich the tirst portion is raised, a regenerator and a bypass connectedto said main in parallelwith said recuperator and ladapted to receivevariable second and third portions of the air carried thereby and raisel the temperature of the combined second and third portions to aconstant level, and a hot blast main connected to said rccuperator,regenerator and bypass -for receiving a combined constant temperaturehot blast therefrom.

7. Anlapparatus as defined in claim 6 including anotl1- er direct-firedrecuperator in said cold blast main for raising the temperature of theair before the iirst,-sec

ond and third portions are taken therefrom.

1,458,288 Gra-ham June l2, 1923 1,816,174

2,514,084 Mowat July 4,l 1950 Brown July 28, 1931`

1. A METHOD OF PREHEATING AN AIR BLAST COMPRISING ROUTING A CONSTANTFIRST PORTION OF A COLD BLAST THROUGH A RECUPERATOR MAINTAINED AT ACONSTANT TEMPERATURE, ROUTING VARIABLE SECOND AND THIRD PORTIONS OF THECOLD BLAST THROUGH A REGENERATOR AND A BYPASS RESPECTIVELY, THERECUPERATOR, REGENERATOR AND BYPASS BEING ARRANGED IN PARALLEL,COMBINING THE THREE PORTIONS TO FORM A SINGLE HOT BLAST, AND RELATIVELYINCREASING THE SECOND PORTION AND DECREASING THE THIRD PORTION AS TTHEREGENERATOR BECOMES PROGRESSIVELY COOLER TO MAINTAIN THE HOT BLAST AT ACONSTANT TEMPERATURE.